This invention relates generally to liquid electrophotographic colored imaging apparatus. More specifically, it relates to an improved developing system for applying color toner from a plurality of individual developing units to a charged latent image.
Original artworks, such as photographs, hand or machine drawn advertising, computer generated images or hand drawn pictures are usually converted from continuous tone to half-tone or dot matrix format to be able to easily reproduce the artwork on a printing press. Color original artworks are generally separated into four half-tone color separations simply known as separations. Each half-tone separation has one of the four colors of the artwork applied to the image thereon. It is possible to convert original color artwork into more than four color separations, which may be achieved in a number of ways known in the art. These may include the use of a camera utilizing different color filters with the illuminated art object, or with a graphics art scanner.
Traditionally, the separations have been formed on silver films and a color proof image, commonly known as a proof, is produced from the separation films and is visually compared to the original artwork. The separations are then judged or compared with the original artwork to determine acceptability of reproduction. A successful color proof then has the image of each separation reproduced by known techniques on a separate printing plate that is typically formed of an aluminum sheet with an organic film coating. The printing plates are then employed to successively print the images onto a receiving substrate such as paper, metal, plastic, or fabric.
Various commercial proof generating equipment is available, such as that offered by the 3M Company under the tradename "Color-Key". This system utilizes pre-pressed proofing materials with pre-sensitized ink pigment coatings in either transparent or opaque colors on transparent polyester base sheets. Each primary color and black base sheet is overlayed by its associated separation negative and after exposure and development the four Color-Key, sheets are overlayed and registered to provide a proof or simulation of what the four-color work will look like when printed.
Another commercially employed pre-press color proofing system is also marketed by the 3M Company under the trade name "Transfer-Key" and is intended to provide a complete four-color proof on a single sheet. In this system, precoated carrier sheets of color pigment bearing the primary cyan, yellow, magenta and black pigments are successively laminated onto the base material by use of a specially developed laminator. This system employs a lamination/exposure/development cycle that is repeated for each color to produce the four-color proof. Each carrier sheet is first laminated to the base material, then exposed to the particular color separation negative, and then developed in a specially developed processor.
A third system commercialized by the 3M Company for color proofing is sold under the "Matchprint II" which is alleged to employ the advantages of "Transfer-Key" proofs, but utilizes a single level of optical gain in order to attempt to accurately simulate the press gain encountered in high-speed printing publication.
Another commercially available pre-press color proofing system is that sold by the E.I. Dupont de Nemours Company under the tradename "Cromalin." This system employs a master film that is sticky on its exposed surface and which, when selectively exposed to light, becomes hardened and nonsticky in the exposed areas. pigment toners are then rubbed onto the surface and adhere in the sticky areas to form a layer of developed image.
Numerous other color proofing systems have been commercially used, such as the Coulter color proofing system that uses a low voltage, high charge density electrophotographic material with submicron resolution and high edge acutance in both analog and digital imaging models. This system employs a cadmium sulfide electrically anisotropic crystalline photoconductor on a metal base. Essentially this same system is currently used by Stork. Another system that has recently been introduced is the Kodak "Signature" system which uses an electrophotographic color proofing apparatus to generate half-tone color separations derived from a piece of artwork and comprises the steps of charging, exposing and developing.
All of these previous electrophotographic systems use a development technique, however, which suffers from the disadvantages of not being able to maintain the same physical distance between locations for the charging of the latent image and its development at each toner development station and not being able to prevent cross contamination of the toners from adjacent toners. Previous electrophotographic systems with multiple toner developing station locations also experience varied levels of drying of the liquid toners on the surface of the photopolymer master or photoconductor between the developing and transfer stations because of the different distances between each of the multiple individual developing stations and the single transfer station along the processing path. Uniform developing times were achievable in prior art systems, but only with variable developing speeds. Additionally, most of the prior commercial color proofing apparatus has been of substantial size and also lacked the versatility and the ability to easily increase the number of colors employed without increasing the floor space.
These problems are solved in the design of the color imaging apparatus of the present invention utilizing an improved development system that has individual development units that are moveable between, a plurality of locations in a first storage position and a single common second operating developing position.